Automatic gain control circuit



Dec. 10, 1946. L1. A. RAJcl-IMAN ET AL y2,412,423 Y AUTOMATIG 'GINfoNTRoL CIRCUIT Filed April 29, 19.45 5 Sheets-sheet 1 Gttorneg Dec. 10, 1946. J. A. RAJCHMAN ET AL 2,412,423

` I AUTOMATIC GAIN CONTROL CIRCUIT FiledApril 29, 1943 s Sheets-sheet 2 5 Sheets-Sheet 3 J. A. RAJCHMAN ET Al.

' Filed April 29, 1943 AUTOMATIC GAIN CONTROL CIRCUIT Dec. 10, 1946.

Patented Dec. l0, 1946 UNITED STATES PATENT OFFICE AUTOMATIC GAIN CONTROL CIRCUIT Application April 29, 1943, Serial No. 485,040

(Cl. Z50-41.5)

Claims. 1

This application is a continuation-in-part of our copending U. S. application, Serial No. 449,244, filed July l, 1942, and assigned to the same assignee.

This invention relates generally to electron discharge devices and particularly to an improved method of and means for controlling the gain of an electron multiplier or other photo-sensitive electronic amplifying device.

One of the principal difficulties encountered in the use of electron multipliers or other photosensitive amplifying devices is that of maintaining the gain of the device at a constant value irrespective of changes in tube characteristics and supply voltages. Richard L, Snyder, Jr. has described one method of gain control for electron multipliers in his U. S. Patent 2,198,233, granted April 23, 1940. The subject patent, however, requires a specially designed electron multiplier for efficient operation, whereas the instant invention is adapted to the automatic gain control of multipliers of all types.

Briefly, one embodiment of the invention contemplates the control of any electron multiplier (for example, of the RCA type 931) which is actuated lby a light beam modulated by some predetermined signal source. The output of the electron multiplier may, if desired, be further amplified and applied in a load circuit. In order to stabilize the gain of the electron multiplier, a second light source, which is modulated by any other predetermined frequency source, is also focused upon the photo-cathode element of the electron multiplier. The output of the multiplier is applied to separate filters. One filter is designed to pass only the signal frequency, while the second filter is designed to pass only the control frequency. The control frequency component is applied to a linear rectifier, and after comparison with a source of reference voltage, is further amplified and stabilized. The stabilized control voltage derived from the last mentioned amplifier is applied to the electron multiplier to control the gain thereof.

A second method of controlling the gain of an electron multiplier utilizes a multivibrator, or similar device, Which alternately applies voltages, from a signal source and a control source, to two light sources which are focused upon the photocathode of the electron multiplier, The output of the electron multiplier is fed into a buffer amplifier, the output of which is connected to the input circuits of a signal amplifier and a control amplifier. The gain of the signal and control amplifiers issynchronously controlled by the same multivibrator, whereby the electron multiplier alternately is responsive to the signal source and the control source. The output of the control amplifier is rectified and compared with a source of reference voltage. The combined voltages are further amplified and regulated, and applied to the electron multiplier to control the gain thereof. A third embodiment of the invention includes the utilization of either of these two methods to control the gain of a photo-electric cell and a photocell amplier.

Among the objects of the invention is to provide an improved method of and means for controlling the gain of an electron discharge device. Another object is to provide an improved method of and means for controlling the gain of an electron multiplier having only one electron stream. Another object is to provide an improved method of and means for controlling the gain of an electron multiplier wherein a control frequency applied to the multiplier is selected, rectified, compared with a source of reference potential, and applied to the electron multiplier to control the gain thereof. Another object is to provide an improved method of and means for controlling the gain of an electron multiplier wherein a control voltage is intermittently generated by the electron multiplier, and compared with a source of reference potential and applied to the multiplier as a gain control voltage. A further object is toprovide an improved method of and means for controlling the gain of a photo-cell amplilier actuated by a photocell.

The invention will be described by reference to the accompanying drawings, of which Figure 1 is a schematic block diagram of one embodiment of the invention, Figure 2 is a schematic circuit diagram of the principal features of Fig. 1, Figure 3 is a schematic block diagram of a second embodiment of the invention, Figure 4 is a schematic circuit diagram of the principal features of the modification of Fig. 3, and Figures 5 and 6 are modifications of a third embodiment of the invention. Similar reference numerals are applied to similar elements throughout the draW- ings.

Referring to Fig. 1', a source of signals l, which may be an oscillator having an output frequency 'fa is connected toa rst light source 2 which is signal output terminals 3.

quency fc, is connected to a second light source 5, which may be of the same type as the first light source 2. The second light source is also focused upon the photocathode element of the electron multiplier 3. The collector electrode circuit of the electron multiplier 3 is connected, through a conventional A.-C. feedback amplifier 5, to the input circuits of a first filter l, tuned to pass the ysignal frequency fs, and a second filter 8 tuned to pass the control frequency fc. The output of the first lter l is connected to the The output of the second filter 8 is connected to a linear rectifier I3. The rectified Voltage derived therefrom is combined with a standard or reference voltage I I and applied to the input of a conventional D.C. amplifier I2. The voltages appliedA to the D.C. amplifier I2 may, if desired, be stabilized by the use of conventional Voltage regulator tubes, such as RCA type Vit-150. The amplifier output is circuits of the A.-C. amplifier and the biasing potentials applied to the electron multiplier; Conventional voltage regulator tubes 3l, 38, 39, 40, I and l2 (for example, RCA type X7R-150), are connected across the various power supply circuits to stabilize the potentials applied to the various electrodes. Y

The operation of the circuit of Fig. 2 is as follows: Signal and control sources of different frequency modulate the light sources 2 and 5 `which are focused on the photocathode of the electron applied to the electron multiplier 3 to control the gain thereof. Y

Fig. 2 is a circuit diagram ofr one practical embodiment in which the collector electrode of the electron multiplier is connected through a coupling condenser I3 to the control electrode of a sistor 22 is connected between the cathode of the second amplifier tube I8 and ground. A grid resistor 23 is connected between the control electrode of the second amplifier tube I3 and ground. A feedback circuit, comprising the series resistors 24 and 25 connected in parallel with the series capacitors 26 and 2, is connected between the anode of the second amplifier tube I8 and the cathode of the first amplifier le. Anode potential is applied through serially connected choke coils lf3 and 44 and a series resistor 45. Screen potential is applied throughV the choke coil i3 and screen resistor 55. lThe anode of the second amplifier tube I8 is coupled, through a coupling condenser 28, to the input of the signal frequency filter "I, which may be of any well known type designed to effectivelyV limit the output at the terminals I! to the signal frequency component. The anode of the second amplifier tube I8 is also coupled'through the coupling capacitor 23 to the input of the control frequency lter 8, which is also of conventional design. The output of the control frequency filter 8 is coupled, through a coupling capacitor 30, to the anode of a diode 3|. The rectified output of the diode 3l is combined with an adjustable D.C. potential derived from a potentiometer 32, and is applied to the control electrode of a D.C. ampliner 33. A source of reference potential 34, which may be a battery, is connected across .the terminals of the potentiometer 32. The output of the first D.C. amplifier tube 33 is further amplified by second and third D.C. amplifier tubes 54 and 35, respectively, and is applied to the terminals of a bleeder resistance 36, which supplies the operating voltages for the electron multiplier 3. Separate sources of potential are preferably provided for the anode multiplier 3. Potentials of both frequencies are amplified by the tubes I4 and I8 and the signal frequency is selected by the filter 'I and applied to the signal output terminals 3. Potentials of the control'frequency are Vselected by thelter 8 and rectified by the diode 3 I. The rectified control signals are then combined with the adjustable bias Voltage derived from the battery 34 and the potentiometer 32 and applied to the grid of the amplifier tube 33. After additional amplification in the D.C. amplifier tubes 54 and 35, the control signals are applied to the multiplier supply circuit to compensate for multiplier variations. Y

In Fig. 3 the signal source I and the control voltage source l are connected to multivibrator 55, which alternately triggers the sources I and 4 to energize the signal light source- 2 and the control light source 5, respectively. The light from the two sources 2 and 5 is focused upon the photocathode of the electrode amplifier 3. The output of the electron multiplier 3 is applied to the input circuit of a D.C. amplifier 5I. The output circuit of the D.C. amplifier 5I is connected to the input circuits of' a second D.C. amplifier 52 and a third D.C. amplifier 53. The output circuit of the second D.C. amplifier 52 is connected to the signal output terminals 29. The switching multivibrator 50 is also connected to the second D.C. amplifier 52 and the third D.C. amplifier 53 to control intermittently-and alternately the gain thereof in synchronism with Y 1 fier 53 is combined with a source of reference Vpotential II in the same manner as in: Figs. 1

and 2, and is applied to the input of a fourth` D.C. amplifier I2. The output of the fourth D.C. amplifier I2 is applied to the electron multiplier -to control the gain thereof. The control .voltage source e, in this modification, may be either A.-C. or D.C. since the signal and control sources are not applied to the multiplier circuit at the same time, and since D.C. amplifiers are used throughout.

Fig. 4 is a circuit diagram of. another practical embodiment of the system in which the electron multiplier 3 is provided with biasing potentials derived from the bleeder resistor 36. The collector electrode of the electron multiplier 3 is connected to the control electrode of a first D.C. amplifier 5I. The anode of the first D.C. amplifier 5I is connected through a first coupling resistor BI to a first control electrode of the second D.C. amplifier 52 and a first control electrode Y to the anode of a diode 3|- land are coinbinedwith the reference voltage, derived fromv thepotentiometer32. The-resulting voltage is ampliedby the 13h-C. amplifiers 54, 35 and 66- in the same mannerV as describedheretofore for the circuit of Fig. 2. The output current of the last D.C. amplifier -tube 66 flows through the bleeder resistance 36, to controll the gain of the electron multiplier 3. Similarly, the potentials applied to the anode circuits andelectron multiplier electrodes are stabilized by conventional voltage regulator tubes such as the type VBL-150, as in the circuit of Fig. 2.

A conventional multivibrator circuit 50 is connected to a second control electrode of the second D.C. amplifier tube 52 and applied` toY trigger the signal source l. The multivibrator is also connected to the second control electrode `of the third D.C. ampliiier 53 V`and applied to trigger the control voltage source ll, whereby the signal and control channels are alternately and intermittently energized and conditioned. The multivibrator circuit 50 may be of any suitable type known in the art, for example, the circuit described in the' Review of Scientiiic Instruments, vol. 9, July 1938, at page 222.

Thus, the operation of the circuit of Fig. 4 is as follows: Modulated light is alternately appliedtothe electron` multiplier 5tv by the keying action of the multivibrator on the signal source l andthe controlV source 4. Simultaneously, the D.C. ampliiiers A52 and 53 are keyed by the multivibrator 5o to synchronize their operation with the signal and control sources. The signal output is derived from the terminals 2Q of the cathode resistor 623m circuit with the amplifier 52.

Control voltages from acrossthe cathode resistor 63 in circuit with the amplifier 53 are. rectified by thediode 3ll and combined with the adjustable bias voltage derived from` the battery 34 through the voltage divider .32.. The combined voltages, which include signals representative of Variations in the characteristics of the multiplier, are amplied and appledto the multiplier supply circuit to compensate for such multiplier variations.

Fig. 5 is similar to Fig. 1, with the exception that a conventional photocell Bl and variable gain amplifier 6.9 are substituted for the electron multiplier 3. The operation is generally the same as that of Fig. 1.

Fig. 6 is similar to Fig. 3, with the exception that the photocell Sl and variable gain amplifier 69 are substituted for the electron multiplier 3. The operation is generally the same as that of Fig. 3.

The variable gain ampliers of Figs. 5 and 6 may be of any type known in the art wherein the gain is controlled by a gain control voltage derived from associated apparatus.

The systems disclosed in Figs. 5 and 6 differ from previously disclosed devices in that the instant systems employ a common photocell and photocell. amplifier for both the control. voltage and the signal voltage components.

We claim as our invention: l. The method of controlling the gain of a electron multiplier device comprising generating separate distinctively modulated light beams, one of said beams providing signal intelligence and another of said beams providing a control signal, applying said light beams to said multiplier to produce a common electron stream therein, deriving from said multiplier combined signals each characteristic of the distinctive features of said light beams, separating said signals, deriving Cil control potentials from one of said Separated signals, and applying said `control potentials to said device as bias voltages to control the velocity of said electron stream. and hence the gain of said multiplier.

2. The method of controlling the. gainof an electron multiplier device` comprisinggenerating separate. light beams. modulated at different` frequencies, simultaneously applying said light beams.` to said multiplier to producev a common electron stream therein, deriving. combined. signals from said multiplier, separating signals of diierent frequency components, deriving control potentials from one of said separated signals, andr applying said control potentials to said device as bias voltages to control the velocity of saidl electron stream and hence the gain of said multiplier.

3. The method of controlling the gain of an` electron multiplier device comprising generating separate light beams, one of said beams providing signal intelligence and another of said beams providing a control signal, alternately applying said light beams to said multiplier,` deriving from saidmultiplier combined signals each characteristic of a different one of said alternately applied light beams, separating said signals, deriving control potentials from one of said separated signals, and applying said control potentials to said device as bias voltages to control the. velocity of said electron stream and hence the gain of said multip-lier.

4'. A circuit for controlling the gain of anr electron multiplier device including means for generating separate distinctively modulated light beams, one of said beams providing signal intelligence and another of said beams providing a controly signal', means for applying said light beams to said multiplier to produce a common electron stream therein, means for deriving from saidl multiplier combined signals each characteristic of thedistinctive features of said light beams, means for separating said signals, means responsive to one of said separated' signals for deriving control potentials therefrom, and means for applying said control potentials tosaid device as bias voltages to control the velocity of said electron stream and hence the gain of said multiplier.

`5. A circuit for controlling the gain of an electron multiplier device including means for generating separate light beams, means for mod'- ulating said beams at diiiferent frequencies, means for simultaneously applying said beams to said multiplier to produce a common electron stream therein', ymeans for deriving combined signals from said multiplier, means for separating said signals, means responsive to one of said separated signals for deriving control potentials therefrom, and means for applying said control potentials to said device as bias voltages to control the velocity of said electron stream and hence the gain o f said multiplier.

6. A circuit for controlling the gain of an electron multiplier device including means for generating separate light beams, one of said beams yproviding signal intelligence and another of said beams providing a control signal, means for alternately applying said light beams to said multiplier, means for deriving from said multiplier combined signals each characteristic of a different one of said alternately applied light beams, means for separating said signals, means responsive to one of` said separated signals for deriving control potentials therefrom, and means for applying said control potentials to said device as bias voltages to control the velocity of said electrongstream andhence the 4gain of said,V multiplier.

A'lcircuit of the type described in claim 6, including means for modulating atleast one of saidlight'beams.

V 8. A circuit of the type. described in claim 5, including means for deriving a gain controlled signal from said multiplier.

9. A circuit of the type described in claim 6, including means for deriving a gain controlled signal from said multiplier.

10. A circuit for controlling the gain of .an electron multiplier device including means for generating separate light beams, one of said beams ,providing signal intelligenceand another of said beams providing a control signal, means including a multivibrator for alternately applying said light beams to said Amultiplier, means for deriving from said multiplier rcombined signals each characteristic of a dierent one of Vsaid alternately applied light beams, means includingV said multivibrator for separating said signals, means responsive to one of said separated signals for deriving control potentials therefrom, and means for applying said control potentials to said device as bias voltages to control the velocity of said electron stream and hence the gain of said multiplier.

11. A circuit for controlling the gain of an electron multiplier device including means for generating separate lightI beams, means for modulating said beams at different frequencies, means for simultaneously applying said beams to said multiplier to produce a common electron stream therein, means for deriving combined signals from said amplifier, filter means for separating said signals, means responsive to one of said separated signals for deriving control potentials therefrom, and means for applying Said control potentials ,to said device as -bias voltages to controlthe velocity of said electron stream and hence the ygain of said multiplier.

12. A circuitrfor controlling the `gain of an electron multiplier device including means for generating separate light beams, means for modulating said beams at dinerent frequencies, means for simultaneously applyingsaid beams to Y said multiplier to produce aV common` electron stream therein, means for deriving combined signals from said multiplier, filter means for separating said signals, meansincluding a source of reference potential and responsive to one of said separated signals for deriving control potentials therefrom, and means for applying said control Ipotentials to said device as bias voltages to control the velocity of 4said electron stream and hence the gain of said multiplier.

13. A circuit for controlling the gain'of an electron multiplier device including means for generating separate light beams, onel ,of said beams providing signal intelligence and another beams, one of said beams providing signalintelligence and another of said beams providing a control signal, applyingsaid light beams to said device to produce a common electronic circuittherein, deriving from said device combined signals characteristic of thev distinctive features of said light beams, separating said signals,

deriving control potentials from one of said lsepi arated signals, and applying said control. pOlenof said beams providing a control signal, means including a multivibrator for alternately apply-v ing said light beams to said multiplier, means for deriving from said multiplier combined signalspeach characteristic of a different one of said alternatelyapplied light beams, means including said multivibrator for separating said signals, means including a source of reference potential and responsive'to one of said separated signals for deriving control potentials therefrom, and means for. applying Vsaid control potentials to said'device as bias voltages to control the velocityV tials toV said device as bias voltages to controlk the Yvelocity of saidl relectron streamand hence the gain of saidY device. l t n l5. The method of controllingy the gain of a photo-sensitive amplifying device comprising generating separatev light beams modulated at diffeiyent frequencies, simultaneously applying said light beams to said device to produce a common electronicV circuit therein, deriving combined signals Vfrornsaid device, separating signals of ,different frequency components, VAderiving control potentials from one o-f said separated signals, and applying said control potentials to said Vdevice as bias voltages to control the velocity of saidV electron stream and hence the gain of said device.

16. The method of Ycontrolling, the gain of a photo-sensitive amplifying device comprising generating separate light beams, one of said beams Vproviding signal intelligence and another oi' said beams providing a control signal, alternately applying said light beams to said device, deriving from said device combined signals each characteristic of a diierent one of saidalternately applied light beams, separating said signals, deriving control potentials from one of said separated signals, and applying said control po-V tentials to said device as bias voltages to control the velocity of said electron stream and hence the gain of' said device.

1'?. A circuit for controlling the gainof a photo-sensitive amplifying device including means for generating Vseparate l distinctively modulated light beams, one of said beams providing signal intelligence and another of said beams: providing a control signal, means for applying said light beams to said device to produce a common electronic circuit therein, means for deriving from said-device combined signals each characteristic of the 'distinctive features of said light beams,

means for separating said signals, means responsive to one of said separated signals for deriving control potentials therefrom, and means for applying said control potentials to said device asV bias voltages to control the velocity of said elec-` tron, stream and hence the gain Vof said device.

18. A circuit for controlling the gain of a photo-sensitive amplifying device including means for generating separate lightbeams, means for modulating said beams at dierent frequencies, means for simultaneously applying said `beams to said device to ,produce a common electronic cir-1 cuit therein, means for deriving combined signals from said device, means for separating said sig-V Inals, means responsive to one of said separated signals for deriving control potentials therefrom,

and means for applying said control potentialsv to said device vas bias voltages to control the velocity of saidelectron stream and hence thev Y gain of said'device.

i9. A circuit for controlling the gain of a photo-s'e'nsitive amplifying device including meansY for generating separate light beams, one of said beams providing signal intelligence and another of said beams providing a control signal, means for alternately applying said light beams to said device, means for deriving from said device combined signals each characteristic of a different one of said alternately applied light beams, means for separating said signals, means responsive to one of said separated signals for deriving control 10 potentials therefrom, and means for applying said control potentials to said device as bias voltages to control the velocity of said electron stream and hence the gain of said device.

20. A circuit, of the type described in claim 19 including means for modulating at least one of said light beams.

JAN A. RAJCHMAN. EDWIN A. GOLDBERG. 

